The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
Mass spectrometry imaging (MSI) is a technique that allows simultaneous chemical and/or molecular analysis of a sample and to visualize the spatial distribution of the sample by their molecular masses.
In an ion microscope mode MSI system, a laser-ionization technique such as matrix-assisted laser desorption ionization (MALDI) or a primary beam of energetic particles as in secondary ion mass spectrometry (SIMS) may be used to ablate/ionize ions from a sample surface. The ions are electro-statically guided by ion optics to a detector system wherein the spatial information of the sample is preserved and the ion optical image is magnified. The full ion load may be detected by the detector system, comprising a chevron micro-channel plate (MCP) stack and a pixelated position-sensitive and time-sensitive detector. The detected position- and time-information enables molecular imaging and m/z identification and analysis. Such MSI imaging configuration may also be referred to as an ion microscope mass spectrometer or—in short—an ion microscope. The whole MSI process is performed in a vacuum chamber, which is kept at a high vacuum pressure.
An exemplary state-of-the art MSI system is described in the article by Jungmann et al., “Fast, High Resolution Mass Spectrometry Imaging Using a Medipix Pixelated Detector”, Journal of the American Society of Mass Spectrometry 21, 2023-2030 (2010). In this system state-of-the-art Medipix detectors are used, which is capable of time-resolved measurements with a time resolution of 10 ns. Four Medipix detectors are arranged in a 2×2 tiled configuration in order to provide an active detection area of around 8 cm2. These pixelated detectors produce large amounts of data (in order of 1 Gigabits/second or more), which require fast read-out electronics. Due to these data acquisition requirements, a substantial part of the readout electronics is mounted in the vacuum chamber close to the detector.
It is highly desirable to extend the use of ion microscopes such as the MSI system described by Jungmann et al to the analysis and imaging of large-area high-mass macromolecular complexes of m/z values larger than 100 kDa (e.g. large protein complexes, viruses and bacterial clusters), which are present in bio-molecular, medical and/or pharmaceutical samples. Prior art MSI system however limited capabilities for analysing such high-mass macromolecular complexes due to a number of shortcomings.
One problem is the so-called “high-mass roll-off” of the MCP detector. An MCP depends on the generation of secondary electrons wherein the generation of the secondary electron avalanche is proportional to the velocity of the incoming particle. As all particles are given the same initial kinetic energy, high mass ions will impinge on the detector with a relatively low velocity/momentum. Hence, these high-mass ions will have insufficient energy for the MCP to generate an electron shower and as a result high-mass ions are not detected.
Increasing the acceleration voltage to higher values may solve the high-mass roll-off problem. However, in the known MSI system the detector is held at ground potential so that the detection system is limited to one-polarity acceleration voltages of about 5 kV. Moreover, precise and accurate imaging of high-mass ions by the MCP/detector combination at high voltages requires ultra-high vacuum, i.e. 10−9 mbar and lower, over the full length of the spectrometer all the way up to the detector side. In the known MSI system however a substantial part of the read-out electronics is mounted in-vacuum in the vacuum chamber close to the detector. For that reason, the pressure at the detector side cannot get lower than about 10−7 mbar because of outgassing of the vacuum-incompatible readout board.
The fact that the prior art microscope mode MSI systems are only suitable for acceleration voltages of one polarity poses serious limitations to its suitability for high-mass applications as in the analyses of high-mass macromolecular complexes the information associated with positive and information associated with negative ions are complementary and crucial for a full analysis the sample.
Extending the known MSI system to high acceleration voltages of both polarities thus poses some serious technical challenges. Hence, there is a need in the art for a detection system, which allows ion microscopy at high acceleration voltages and ultra-high vacuum conditions. Further, there is a need for a high-voltage detection system, which allows both positive and negative mode ion microscopy.